Google struggles to keep Christchurch gunman's killing spree off Youtube

YouTube has worked to block certain videos from ever showing up on its site for years

YouTube has tried to keep violent and hateful videos off its service for years. The Google unit hired thousands of human moderators and put some of the best minds in artificial intelligence on the problem.

On Thursday, that was no match for a gunman, who used social media to broadcast his killing spree in a New Zealand mosque, and legions of online posters tricking YouTube’s software to spread the attacker’s video.

When the rampage was streamed live on Facebook, police alerted the social network, which took the video down. But by then it had been captured by others, who re-posted